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Topic: 24 bit v 16 bit (Read 4818 times)

> The -18dBFS is the EBU standard and the -20dBFS the SMPTE standard to set for 0VU as I understand it - which is why I quoted those figures.

John, I know that you know this, but for the benefit of those who don't:

VU meters are a very particular thing. Not every analog meter is a VU meter--not even every analog meter that has "VU" printed on its face. True VU meters are/were highly standardized "volume indicators" (VU = "volume units"). They were carefully designed in the early 1940s by Bell System engineers to give an idea of perceived loudness, mainly of speech transmission for telephone systems and AM broadcasting. The motion of a VU meter's needle has "syllabic response" with an integration time of 300 milliseconds, which is extremely long by today's standards.

True VU meters are dead-on accurate when fed continuous tones. But because of the long integration time, signal peaks occurring in live, uncompressed program material are typically about 8 dB higher than what you would see on the meter. (John, you will probably recall that PPMs were calibrated so that their zero point was effectively equal to +8 VU for this reason.) With some program material a VU meter would "under-read" signal peaks by a significantly greater amount--especially strong signal components that rise and fall quickly, such as percussion picked up at close range with condenser microphones.

Now, the scale on a VU meter doesn't stop at 0 dB; it continues to +3. The zone between 0 VU and +3 is marked in red, and with analog tape there was higher distortion above 0 VU than below it, sometimes audibly so. But even +3 VU wasn't a "brick wall" limit. The most conservative, purist classical approach to recording still involved "going into the red" sometimes--just not hanging out there for any length of time. Rock music, on the other hand, was often recorded with the needle well into the red a lot of the time--intentionally using tape saturation as a kind of compressor. I've seen some engineers "push" tape so hard that the VU meters were continuously "pinned", i.e. off the scale and all the way to the right, and they were proud of it.

In summary: For live recording, if you set -18 dBFS = 0 VU on continuous tone, your typical (often-recurring) peaks on the digital side will tend to be around -10 to -8 dBFS, with occasional peaks going maybe to around -5 or in truly extreme cases, a touch higher. Those would be very good levels for 24-bit recording in my opinion. So the standards you mentioned make excellent sense IF the fundamental difference between VU meters and peak-reading meters is well understood.

Fewer and fewer people nowadays have ever used real VU meters for live recording, though. So some people infer (wrongly) that their peak digital recording levels should be set to occur at -18 or -20 dBFS. That's not at all what those standards mean. Setting your levels that low is just asking for extra noise--if not from the recording channel itself, then from everything that comes before and after it.

Because some recorders have a high noise-floor and are effectively only recording around 16bits of actual dynamic range even when set to 24bit mode. In that case you'd just produce a larger sized file with no benefit. But to know if that's the case or not you'd have to measure, and since the difference in file size going from 16 to 24 bits isn't astronomic, it's a reasonable choice to record in 24 bit mode without worrying too much wasted file space. < Makes sense to worry more about setting levels (go a bit higher).

In contrast, going from 48kHz to 96kHz doubles the file size, and going to 192kHz quadruples it. That's way more file size bloat than going from 16bits to 24bits and I'd be more hesitant to do so unless I'd convinced myself that the result was worth it.

I ran super hot levels when recording to DAT, the only way thing I hate about 😈 is having to guess at levels and having to add a bunch of volume, yeah I’m doing solid state 24 bit, but I’m losing quite a bit more than I gain by not being able to run hot. Imo.

Have that 620 down now. If I am hitting -12 levels I'm good with it, which generally results if I run 620 levels at -11 or so. So I could probably be fine at like -6 but still don't like to take that chance of clipping. I was worried I clipped Kansas, but I get it home and it's like -12.